Erythroid differenfiafion is controlled at the transcripfional level by the cell-specific DNA-binding proteins SCL and GATA1. Using zebrafish genefics, we found a new differentiation checkpoint in erythroid cells that likely ensures successful maturafion. We undertook the flrst recessive suppressor screen in vertebrates to find a novel mutant that can recover erythropoiesis in the background ofthe anemic zebrafish mutant moonshine. Moonshine encodes the zebrafish ortholog of transcripfional intermediary factor 1 gamma (TIFly), and the first suppressor of moonshine encoded cdc73, a chromatin factor involved in transcripfion elongafion. Moonshine mutants have paused blood-specific RNA transcripts, and cdc73 deficiency allows elongafion to occur and rescues erythroid gene expression. TIFly binds to the SCL transcription factor complex and PTEFb, the kinase that phosphorylates pol II and regulates the elongation of RNA transcripts. We have examined the hematopoiefic phenotype of a condifional mouse knockout for TIFly. Bone marrow erythropoiesis is defective, myelopoiesis is increased, and there is a severe deficiency of B cells. We plan to evaluate transcripfion elongation in erythroid and myeloid progenitors of this mouse mutant. Transplantafion experiments are planned to evaluate a putative stem cell funcfional defect and the autonomy ofthe red cell defect. We will establish if the differenfiafion checkpoint is acfivated in diseases such as hypochromic anemias or membrane assembly defects. HEXIM 1 was originally identified as a gene induced during MEL differenfiation, and it complexes with other proteins and 7SK RNA to regulate P-TEFb activity. We hypothesize that HEXIM complexes could serve as a sensor of erythroid differenfiafion. We plan to define the binding partners of HEXIM in erythroid cells, and to evaluate the funcfion ofthe HEXIM complex subunits during erythropoiesis in fish and mice. The function ofthe 7SK RNA component of HEXIM will be invesfigated. A new sequencing technology will determine if other small RNAs are bound to the HEXIM complex. The regulation of transcription elongation in the differentiation checkpoint of erythroid cells wili be relevant to diseases such as thalassemia, sickle cell anemia, iron deflciency, and membrane defects.